Distinct tube size is critical for the function of human tubular organs such as the lung, vascular system, and kidney. Aberrant tube sizes that arise during development can lead to devastating human illnesses such as Meckel kidney disease and polycystic kidney disease. However, the fundamental mechanisms that regulate tube size are still not well understood, preventing us from developing effective treatments for diseases caused by tube-size defects. Drosophila trachea provides a premier genetic system to investigate the fundamental mechanisms that regulate tube size. Mechanisms of tube-size control discovered in Drosophila trachea can be subsequently investigated in vertebrates, thereby extending our understanding of tube-size regulation in tubular organs in general. Recently, through a targeted RNAi knock down survey, we identified Expansion (Exp), an evolutionarily conserved and novel Smad-like protein, as an essential regulator of tube size in Drosophila trachea. Smad proteins generally function as mediators of Transforming Growth Factor-B (TGF-B) signaling. TGF- B signaling is known to control cell numbers within zebrafish cranial vessels, thereby regulating vessel diameter (Roman et al., 2002). However, our preliminary data indicated that TGF-B signaling and cell numbers were not involved in tube-size defects in exp mutants. Instead, the apical secretion of luminal proteins was defective, and the tracheal apical membrane was larger in exp mutants. In addition, phosphorylated MAP kinase (dp-Erk) levels were increased in exp mutants. Moreover, reducing Erk expression in exp mutants suppressed the observed tube-diameter defects. Other groups have shown that Epithelial Growth Factor (EGF) signaling phosphorylates Erk in Drosophila (Ohshiro et al., 2002) and that over-active EGF signaling expands tracheal tube diameter (Jeon et al., 2009), similar to our observations with exp mutants. Therefore, we hypothesize that Exp reduces EGF signaling to control the dynamic cellular processes required for tube-size regulation. We will test this hypothesis through the achievement of two specific Aims: (1) Identify the cellular processes that are regulated by Exp; 2) Determine how Exp regulates EGF signaling to control tube-size. Through the proposed experiments, we expect to define a novel role for Smad family proteins in regulating signaling pathways and downstream dynamic cellular processes to control tube-size. We expect that the results from the proposed study will advance the understanding of how tubes acquire their distinct sizes in tubular organs in Drosophila, and many mechanisms will be shared with other organisms, including vertebrates. Ultimately, such knowledge has the potential to provide novel targets for developing treatments for human diseases caused by tube-size defects.